Grounded grid microwave modulator



N0V 21, 1950 w. w. MUMFORD GROUNDED GRID MICROWAVE MODULATOR 2 Sheets-Sheet 1 Filed July 1, 1949 UU@E Nov. 21, 195o W. W. MUMFORD 2,530,836

GROUNDED GRID MICROWAVE MODULATOR Filed July l, 1949 2 Sheets-Sheet 2 /oa/ A FIG. 4 p92 //4 l v I70 0 IW f /NvE/vm@ By W W MUMFORD )7. d

Patented Nov. 2l, 1950 GRGUNDEB GRID MICROWAVE MODULATO'R William W. Mumford, Atlantic Highlands, N, J.,

assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application July 1, 1949, Serial No. 102,492

9 Glaims.

This invention relates to microwave apparatus and more particularly to microwave modulators employing electron discharge devices for .operation at frequencies in .the order of several thou.- sand mega cycles and higher.

The invention aims at improving the operation of microwave vacuum tube modulators, or converters, used `to raise signal energies to a desired microwave frequency band.

A general object of the invention is to improve the structure of microwave vacuum tube ymodulating apparatus and in other respects .to eiect improvements in systems and apparatus of this general character.

A 4more specific object of the invention is to conserve or utilize most eifeetively the ultrahigh frequency Wave power involved microwave systems 'employing thermionic discharge devices as the modulating element.

A modulator in .accordance with the inventiOn and hereinafter to :be described in :detail includes a grounded grid triode having a nonlinear Cath.- cde-voltage :plate current characteristic. The grid of the tricde forms a `common boundary of a pair of resonant chambers ror cavities which are coupled to an input and .anoutput wave guide, respectively. Beating oscillations are impressed .on the input Vwave :guide coupled to the input cavity for intermodulaton with waves of 4sig-nal frequency impressed on the Acathode T.of the triode with the resulting modulated sidebands cone veyed .to the output waveguide.

A feature of .the invention is the use .of :a re nection type filter the input wave guide to preserve the `power content .of the .modulated ,sideband energy appearing in the input cavity by preventing its .absorption inthe beating OSCllaOI branch of .the modulator. The iilter is positioned :in the input'wave guide .so .as to reflect the -modulated sideband energy appearing in the input cavity into the output :cavity ,in phase lwith the modulated :sideband energy appearing therein,

Another :feature resides in the .use of a `coaxial coupling circuit `for :coupling the v,output cavity to .the output wave guide. The .coupling circuit p includes .a movable quarter-wave coaxial vtransformer which forrnsa ypart of "the boundary of 'the output cavity and serves .also :for vmatching and tuning purposes.

AA more .particular ff'eature `of the invention relates to an externally adjustable tuning mechanisrn associated with @the coaxial fcouplingcircuiit and arranged to `vary the position of the quarter- Awave 'transformer :and thereby affecting the :tuning ofathe output cavity.

The invention is further characterized by trap and lov-pass `circuits to isolate the input and .output energies from various parts of the modulator thereby improving the .Conversion gain and eiiciency of the system, A maximally flat lter in the output wave guide selectively transmits waves of the wanted sideband frequency only and reilects the unwanted energies back into the modulator to conserve and translate their power content into wanted sideband power.

Other objects, advantages and features of the invention will be apparent from the `following der .tailed .description and drawings and from the apr pended claims:

Fig. l illustrates schematically the equivalent circuit of the modulator of the present invention;

Fig. 1A illustrates schematically an alternative form 0f .Cathode biasing arrangement .that may be employed in the circuit .of the present invention;

Figs. 2 and 3 are side and front sectional eleva,- tions of e, preferred forni of the modulator to be described herein, the latter ligure showing the driving mechanism vfor tuning the output cavity;

Figs. 2A and .3A are .details of a pair of members of the housing assembly; and

Fig. 4 illustrates a modulator using alternative forms of coupling and trap circuits.

Fig. 1 schematically illustrates one method of using a triode as a modulator. Beating ,oscillator l0 of frequency f2 and signal source Il of fre.- quency f1 are each impressed on the .cathode ,I3 of .a triode l2 having a grounded grid I4. The tube is operated over a non-linear portion of its cathode-voltage plate current characteristic and the resulting modulation products including the sideband frequencies (fzifi) `are obtained in .the plate circuit. Other ccnipOnents of energy in the plate .circuit include the frequencies f2, f1, 2f?, 2jr, (f2-1521i), etc. Due to the inherent feedback of a grounded grid circuit arrangement these frequencies will also appear in .the input or the cathode circuit together with the sideband frequencies (fz- P:Ji). By-pass kcircuits I 8 and 19, tuned to the beating oscillation and signal frequency f2 and f1, respectively, are yprovided to prevent the ow of beating oscillator power in lthe sign-al circuit and vice versa. Input ril-ter 2U tuned .to the sideband frequencies (hifi) prevents absorption oi the modulated sideband energy by the beating oscillator and .concentrates .the ,sideband power inthe output circuit thereby increasing the eiciency and improving the conversion gain of the modulator. `Output lter 22 prevents 4transmission of all but the wanted .side- .band ift-fluency (iM-f1) in the :output circuit.

- reach the output wave guide 1i).

Fig. 2 illustrates in cross-section a side view of a preferred embodiment of the modulator. The enclosure block or housing assembly containing the modulating element and coaxial coupling circuit is constructed of suitable conducting material such as brass and is composed of several rectangular sections including a bottom block and a central block 29, each of the latter having a cylindrical bore extending centrally therethrough. The cylindrical space within block 29 opens into a rectangular cavity formed by top block 3l and its removable cover plate 32 and affords a path whereby the output energy may Rectangular cavity 3i! is of the same cross-section as the output wave guide. A metal can 33 is fitted to the bottom of block 25 to accommodate the terminal connection leads, by-pass condensers 34 and transformer 55 for the modulating element.

Rectangular plates 26 and 21 are interposed between blocks 25 and 28. Plate 25 is a thin U- shaped element as illustrated in Fig. 2A and forms a part of the input cavity 49. The open end of plate 26 forms a rectangular aperture 55 extending partially across one side of the housing assembly (see Fig. 3) and serves to admit microwave energy from the input wave guide 51 upon the modulating element 35. Plate 21 is shown in detail at Fig. 3A as a thin rectangular block with a cylindrical central bore and a square slot across its upper surface to accommodate the loading resistances 8| of Fig. 3 which may be inserted into the plate circuit cavity to provide for a limited adjustment of the band width of the circuit as described hereinafter.

The modulating element 35 employed in the modulator is an ultra-high frequency, closespaced, parallel-plane microwave triode and may be of the Western Electric type 416A or 1553, for example. These tubes may operate as amplifiers at a plate voltage of 200 volts and a plate current of 30 milliamperes and as modulators at a plate voltage of 200 volts and plate current of 14 milliamperes. A description of the 1553 triode appears at page 171 of the April 1949 issue of Electronics published by McGraw-Hill in an abstract entitled New Microwave Triode. Briefly, the elements of the triode comprise a cylindrical anode 31, a nely spaced wire grid 38 and an inverted cylindrical cup-shaped cathode member 49 shown encompassing heater elements 42. The anode terminal 39 is an extension of the cylindrical anode 31 into the space within block 28 through the glass seal 43 enclosing the grid and anode elements of the tube. The cylindrical cathode member is enlarged where it extends downward parallel with the metal shell 44 of the tube forming a capacitance 45 (shown schematically) therewith. The microwave input energy passes through the glass seal 46 between grid flange 41 and metal shell 44 to effect the modulation process within the tube.

The grid flange 41 of the triode may be soldered into a recess in the lower face of plate 21 and separates the cylindrical interior of the housing assembly into lower and upper chambers 49 and 59, respectively. The metal shell 44 of the tube forms a part of the lower chamber or cathode cavity 49 and is secured therein by means of the springy contacts 52 circumferentially disposed about and fixed in the bottom of the chamber. The heater and cath-ode leads are brought out thro-ugh eyelets in the base of the tube to a standard connector (not shown) in connecting can 33,

4 The heater leads are by-passed to the connecting can 33 through condensers 34.

U plate 2G provides in addition to the rectangular coupling orifice 55 a short interconnecting low impedance passage 55 of the same cross-section as the coupling orifice leading from the input wave guide to the cathode cavity. As shown in Fig. 2 the height or Vertical dimension of passage 55 is considerably less than that of the input wave guide 51. In a particular instance in which the internal wave-guide dimensions were '7/8 by 1% inches, the passage dimensions were 1/8 by 17/8 inches. The impedance discontinuity presented to the iiow of waves from wave guide 51 into the modulator input cavity 49 by passage is compensated for by means of tuning screw 53 located at the tcp of the enclosure block and extending down into the coupling orifice 55 centrally across its opening. 'Ihe tuning screw introduces a variable capacitance which can be adjusted to improve the match between the input wave guide and the cathode cavity over a 500- megacycle band. Since the length of the interconnecting passage 56 is short compared with the electrical wavelength, its presence may be considered either as a portion of the cathode cavity or as a portion of the coupling iris, 55. In either case the timing screw is provided to establish an impedance match between the cathode cavity and the input wave guide.

In those cases where it is desired to obtain an impedance match over a, very wide frequency band a broad match may be obtained by means of an input tuner (not shown) located in the input wave guide between lter 69 and the enclosure block. A tuner of this type is described by C. F. Edwards in his article entitled Microwave Converters appearing in the Proceedings of thc I. R. E., volume 36, No. l0, for November 1947. This tuner is, in effect, two variable shunt tuned circuitsapproximately an eighth of a wavelength apart in the wave guide and is capable of tuning out a residual mismatch corresponding to six `decibel standing wave ratio of any phase. Each variable shunt tuned circuit is made up of a nxed inductive post and 9, variable capacitive screw located off center in the wave guide.

Reflection lter 50, described below, is shown at the end of the guide 51 coupled with the cathode cavity through coupling orice 55. A suitable ultra-high frequency source 52 of radio frequency power, shown schematically in Fig. 2, supplies beating oscillations at a frequency f2 of 3905 megacycles, for specific example, to the cathode cavity by way of the input wave guide 51 and coupling orifice 55.

Source 64 impresses intermediate frequency signals f1 in the range of 65 megacycles, for example, on the cathode member 45 of the triode through an intermediate frequency impedance transforming network 55 for intermodulation with the beating oscillations impressed upon the input cavity. The impedance transforming network 65 is contained within connecting can 33 and proposes to match the input impedance of the modulator to the signal source 54, which drives the modulator, in order to eliminate any degradation of the system performance due to impedance mismatch. The design of the network 55 takes into account the capacity of the tube and of by-pass capacitance 45, the resistance of the electron stream, and the lead inductanc@ measured at the base of the tube. A particular network which lends itself readily to the existing 15 tube Glllt is the simple T network equivalent of parallel resonant coupled circuits `illustrated at Fig. 2.

A Iconventional :biasing arrangement consisting of :a battery tian'd a low resistance potentiometer l'Il (as indicated schematically inFig. l) may -be connected to terminal 23 in Fig. 2 to supply the appropriate 4positive potential to :the cathode 1.3 thereby "maintaining the grounded grid Ul ruegative with respect to the cathode. rlhis cathode ibiasing arrangement :maintains vthe cathode ait a relatively :constant positive potential. 'The ldirect-current plate current then `varies with the :level-of the :beating oscillator vpower.

Fig. 1A illustrates a modification :of the `,cathode .biasing arrangement :oi Fig. 1 which has been .found to :hai/ e a greater fstabilizing -eiiect on the direct-currentiplate current. 'This network rmn-ifmizes :small 'variations :in gain due to 4:power line yariations and other disturbing 'influences by ipro lducing .a large amount of negative :feedback lior :the :direct :current path. jlnthis :arrangement :the inegative terminal 'of the .E+ plate :supply is connected to ground throughe battery :16' of .opposite polarityl .The cathode i113 of the grounded rgrid :triode is connected to the junction yof :the B terminal and the battery k6 through sa trar :iable resistance flil. z.The space current to the cathode causes fa potential fdrop 4in the variable resistance "which tends yto :cancel the :voltage of the battery :11.6, thereby maintaining the cathode potential nearer :to rground (.gridl) potential. Any change .in the l'evel of the lbeating 'os cillator power 'which tends "to lchange :thesspace current will tend to :change Llilcewise :the cathode potential in such a sense as to 4counteract the change inspacecur rent. 'The space :current is fthereby held rnearly :constant .at .a .value :determined 'by the cathode `resistor I1" :and the characteristics `of the tube. This space .current fea-n be adjusted to the desired operating uvalue by varying the lcathode resistor.

r.Capacitance 115, mentioned earlier, acts -as a portion .of :the impedance matching network 5% through which .the intermediate-'frequency signal power is applied to 4cathode il when :the tube is rused as a :modulator and .also serves as a radio :frequency trap Ito isolate the microwave energy :trom the tsigned circuit lby ellective'l-y grounding 'the cathode :at .the beating oscillator frequency f2. 'The 4signal lenergy is isolated from ythe beating `oscillator branch :of fthe modulator by lwave guide f :5J whose .dimensions prohibit the propagation Yof signal ener-gy therethrough 'The aforementioned reection lter til located in the input wave .guide rtfl `prevents, the modulated energy rappearing in the input cavity from 1 ilowing into -'the yinput guide where it Vwould be `absorbed by 'the 'beating oscillator. 'It serves lto reflect theirnodulatedenergyfback into the output cavity whereit `will be utilized to best advantage, 'viz.,1to increase power output. The spacing of filter 161i l.along the input wave guide `is so xed itha't the energy reflected -in'to the output cavi-ty `will be in phase with the modulated energy appear-'ing therein.

Filter 60 :may assume the 'form of aband irelieo- `.tion ilter which reilects only 'the Wanted sideband :and passes all other '.reouencies, yor .a :band-'pass filter which passes onli7 the beating oscillation frequency and reflects all fothers. `Whatever `type filter is employed, .it shouldghave -a high standing -Wave ratio at the desired .-sideband frequencies. Fig. 2 .illustrates the use of :the former or band reiection-type lter .employing resonant "rods 61 with adjustable coupling 6B andtuning screws ntil. The structural features 4and performance :of va lter of 'this type are shown 4and :described .by W. D. Lewis and LJC. Tillotsonin theBell System Technical Journal, vol. XXVII for January :19,48 .at page 9:1, Fig. 8. 'Ifhe band-.pass filter may :be of the resonant iris type, the performance and design considerations of which are described inthe above publication at pages 584-713 in 4an yarticle entitled lViaximally-ilat ,Filters in Wave Guide by the present inventor.

Turning now to .the output circuit of the `:modo-- lator, the output chamber or plate circuit lcavity 59 is coupled tothe output wave guide 'lll through `a coaxial conductor couplingcircuit vextending through .the .fcentral space iin block .2.8. The upper portion of the coaxial coupling circuit `includes a coaxial line formed vby the cylindrical interior voi block 'Zit by :an inner conductor i8 which terminates in a wide band coaxial-to-wave-guide transducing probe S3 located .in rectangular cavity 35 Vin the top of the housing assembly. The lower end oic the coaxial coupling circuit includes a quarter-wave resonant section of coaxial .line comprising .a hollow cylindrical slug or Contact member 1:3 surrounding or in juxtaposition with a tubular central member '54.

Slug member 'i3 is slidably mounted within 'block .2-8 and in contact therewith by means of conductive contact ngers at its extremities, and its yinner face together with the .cylindrical Awall of block 28 forms the outer conductor 1'! l .of the coaxial coupling circuit. The .outwardly pressing .spring contact fingers at .the extremities of slug i3 should insure not only .a low resistance electrical connection with the Wall of block 28 but should also provide a complete `closure so yas to minimize losses due to electromagnetic `leak- .age `radiation past slug 1.3. The tubular central member l, of slightly lesser diameter than the surrounding slug "E3, has slotted spring fingers at its extremities to clutch or engage anode connection 33 and the conductor T8, thus forming the inner conductor assembly of the coaxial coupling circuit.

Slug member 'i3 is so proportioned `as to be a quarter wavelength long -at the frequency 'of the desired sideband, 3970 megacycles. rIhe tubular central member 'ifi is slightly longer than slug member 'i3 so that notwithstanding `its movement longitudinally on anode connection 3S 4and conductor "i8 by means presently to be described, it continues to form with member 'i3 fa quarterwave resonant section oi coaxial 'line which serves as an impedance transformer.

The lower surfaces oi the elements 'i3 and "I4 of the impedance transformer are radial and form a boundary of the resonant pill-'box plate circuit cavity 5d. The output or plate circuit cavity 5E! is dened by the grid liti, anode 3l' and anode connection .'39 oi the microwave triode, by the radial faces of the elements of the impedance transformer and by va portionof the interior Wall of block 28 forming the outer conductor H. The plate impedance of the tube is transformed by the plate .circuit cavity to a very low resistance in the order oi" a fraction of an ohm (0.5- 0.7 ohm approximately) on the plate connection 3S just outside the seal t3 of the tube. The quarter-wave transformer matches this low impedance lto the surge impedance (45 ohms approximate'ly) of the coaxial line above the impedane@ transformer. The relative diameters of the elements "I3 and M of the quarter-wave trans.. former are proportioned vso as to `present a ,characteristic impedance equal tothe geometricmean of 4the impedance of 'the plate circuit cavity ,and

thev surge impedance of the coaxial line.

In a particular instance the inner diameter of slug member 'i3 was 0.478 inch. Excluding the Contact lingers, which are one-eighth inch in length, the solid section of E3 corresponding to the quarterwave dimension of the transformer measured 0.727 inch. The outer diameter of tubular member 'I4 measured 0.438 inch and its overall length was 1.00 inch. Figs. 2 and 3 are drawn approximately to scale to indicate the relative proportions of the modulator components.

A limited adjustment of the band width of the modulator may be obtained by introducing external loading resistances into the plate cavity 50 through a small square aperture located on each side of plate 21 of the housing assembly. A simple screw mechanism 99 (see Fig. 3) adjusts the penetration of the loading resistive strips 8| into the plate cavity so as to vary the Q of the plate circuit cavity to match the Q of the output filter section 93 described below.

The inner conductor 'i8 of the coaxial line above the impedance transformer is supported by a dielectric washer 92 secured in a recessed entry in the upper surface of block 28. Conductor 'i9 is undercut where it passes through dielectric washer 82 in order to compensate for the discontinuity introduced in the path of the waves by the dielectric thus assuring an electrically smooth system. The upper portion of conductor 19 extending into the rectangular cavity is threaded to take the coaxial-to-waveguide transducer 93 which an internally threaded hollow probe antenna screwed securely on conductor 7S and against dielectric washer 82.

A hollow stud Se and lool: nut located at the top of the housing assembly is screwed down into the rectangular cavity 39 forming a variable capacitance with antenna probe 83 and is used to adjust the impedance match between cavity 3G and output wave guide '10.

The impedance match looking back into the modulator from the output wave guide lll is broadened by adding the elements of a maximally-at filter 93 in the output wave guide. This lter should not only provide a broad impedance match at the desired sideband frequencies but it should also provide discrimination against the other frequency components present in the output circuit of the modulator. The filter employed for this purpose incorporates the impedance of the plate circuit cavity and may, like the beating oscillator filter 90 in the input wave guide, be of the type whose design and theory are set forth in the aforementioned article entitled Maximally-flat Filter in Wave Guide.

The application of plate voltage to a modulator of the present design raises the problem of providing emcient means to prevent the escape of the modulated energy through the B+ plate supply line. For this purpose a radio-frequency trap is provided in the output circuit of the modulator as shown in Fig. 3. The trap comprises a high impedance quarter-wave wire 86 which is electrically essentially a quarter wavelength at the modulated sideband frequency and functions as a quarter-wave stub across a transmission line presenting a high impedance thereto. The B+ plate supply line 88 is brought through a metallic bushing 99 surrounded by a hard rubber insulating bushing 9! in a side wall of top block 3l and attaches to the termination of the quarter-wave wire 86 in bushing 99. The other end of wire 86 is soldered to antenna probe 93 which makes connection with the anode of the triode through the inner conductor assembly of the coaxial coupling circuit. A button capacitor 99, such as the Erie type 370 button silvered mica condenser, mounted in bushing serves as an insulating support for wire 8S passing centrally therethrough and also as a by-pass to ground for any radio-frequency signal energy that appears on wire 86.

Fig. 3 is a front elevation sectional View of the modulator of Fig. 2 excluding the modulating element 35 and it further illustrates the tuning mechanism that is used to adjust the size of the plate circuit cavity.

The size of the plate circuit cavity may be adjusted by Varying the position of either the central member 'I4 or the slug contact member 'I3 of the coaxial transformer bounding the plate circuit cavity. Fine tuning is accomplished by sliding the central member 14 up and down on the anode connection 39 while a coarse tuning process may be effectuated by varying the axial position of slug member '13.

In the present case the driving mechanism employed to accomplish a fine tuning process is contained within the central member T4 of the coaxial transformer thus effectively isolating the mechanical from the electrical design problem.

The tubular central member 'I4 of the quarterwave coaxial transformer is composed of two parts, a lower sleeve member and an upper shell member 96 each of which have conductive contact lingers at one of their ends to engage anode connection 39 and inner coaxial conductor 78, respectively. Upper shell member 96 is cupshaped, its base having an internally threaded hole axially disposed therein to accommodate the threaded end of a rod 98 passing centrally through 18. The lower portion of the periphery of shell 96 is undercut or slightly recessed, and sleeve member 95 is securely fastened and soldered thereon. The spherical head 99 of rod 90 is retained within antenna probe 83 to prevent longitudinal movement of 98 relative to 18 while allowing rotational movement. Conductor 70 is stationary, being secured between dielectric washer 82 and probe 83. Guide pin i90, fixed in the lower end of stationary conductor '19, passes loosely through a small hole in the base of upper shell member 96 and prevents circular motion of the coaxial transformer central section lll. An actuating tool in the form of an insulated screwdriver 94 or the like is inserted through the hollow stud 84 in cover plate 32 and engages a recessed socket in the head 99 of rod 98. The screw-driver is made of a low-loss plastic material having a dielectric constant slightly greater than unity and is permanently maintained in position. The length of hollow stud 84 through which the screwdriver is inserted provides a length of wave guide beyond cut-off which keepsthe microwave energy from leaking out through the screw-driver. Thus by turning the screw-driver 94 the circular mo tion imparted to rod 98 is translated into a longitudinal axial movement of the central member 14 of the coaxial transformer on inner conductor 'I8 and anode connection 39 and the line tuning process is accomplished.

By means of the ne tuning process the plate cavity resonance frequency can be adjusted to any desired value within a fairly wide frequency band such as, for example, over a SOO-megacycle band, from 3650 megacycles to 4250 megacycles. The position of the coarse tuning slug 13 determines the central frequency of the desired operating; range of frequencies, and this'` position. is adjusted andy fixed at the time when the circuit is assembled so that the desired tuning range is covered by the netuningcontrol. For this purpose slug 13 is; provided with two'tapped' holes. 16 inits upper face to allow for the insertion of threaded screws (not shown)` into. it through holes 81 provided in the dielectric supporting washer 82.. To facilitate the` insertion of' these screws,V the top plate 32: of the cavity 30. is. removable. from the block. 3l thusl making it un.- necessary to disassemble completely-the. circuit to make the coarse adjustment..

Where. the desiredv operating range offrequencies exceeds the. SOO-megacycle band which can be. covered bythe fine tuning process the fol-- lowing: method of moving the slug T3 can be adopted. A. mechanism for sliding the slug can be inserted through the walls. of block 28 withouty aecting the electrical performance of the circuit since the outside of the slug 'I3 and the inside of block 2'8A adjacent thereto. are not. a part of the radio-frequency circuit. This mechanism might include. a pin which engages a slot in slug lf3, the pin bei-ng mounted eccentrically on the end of a cylinder which is free to rotate in block 28. AA screw-driver or wrench may be used to rotate the cylinder from outside of block 28, so that as the cylinder rotates the eccentric pin moves up or down thereby shifting the relative position of the slug.

Fig. 4 is a sectional side elevation of another embodiment of the invention structurally related to the modulator of Figs. 2: and 3 and illustrates the use of diierent types of radio-.frequencyinput and output trap circuits.

Briefly the modulator of Fig. 4 comprises a rectangular enclosure block 28 having a cylindrically bored interior separated into a lower and upper chamber, 49 and 50, respectively, by the grounded grid 38 of the microwave triode electron tube 35. Beating oscillations from a microwave frequency source (i4 are impressed upon the input wave guide 51 coupled to the input chamber 49 by means of constricted rectangular passageway 56 between the chamber and the face of the enclosure block. Tuning screw 5.8 centrally located near'the. entrance `5.5 to. passageway 56 adjusts the impedance match between the input wave guide and chamber 49. A reection-type lter 60 is positioned in the input wave guide as set forth hereinabove. Signal energy is impressed on the cathodel of the tube as shown in Fig. 2.

The input circuit of the Fig. e modulator is thus seen to be similar tc that of Figs. 2 and 3, the principal difference as described below residing in the form of radio-frequency trap employed to isolate the beating oscillator energy from the signal input circuit. in the foregoing description it will be recalled that the intercapacitance between the cathode le and the shell lift oi the tube was utilized to effectively ground the cathode at radio frequency. in the present case the same effect is obtained with a simple quarterwave shorted line its circumferentially disposed about the bottom of the input cavity. Line i515 presents a very high impedance to the micro.m wave energy at its open end as at les and a very low impedance at the top of the sheli et,

thereby isolating the microwave energy from the signai circuit, It has been noted that where the radial dimensions of the quarter-wave trap Edin' become great undesirable higher order transmission modes may be supported. The form 10 of radio-frequency input trap characterized. by condenser [i5 inside the tube in Fig. 2 should be more eiective at higher frequencies than the trap of Fig. 4 since the radial dimensions of the former are less.

The output chamber 5@ of the Fig. 4 modulator is bounded. by the grid 38 and anode 3l of the microwave triode, by the inner wall of' the enclosing block ES and by the radial face of the transformer element 'i'l formed by expanding the diameter of the inner conductor 'i8 of the coaxial coupling circuit. Transformer element 14 may be constructed so as to be slidably mountable on the inner conductor 'i3 and the cylindrical extension 3.9- of the anode electrode for purposes of adjusting th'e size of the` output chamv ber. The outer conductor of the coaxial coupling circuit is formed by a tubular conductive shield 'il of the same internal diameter as the cylindrical bore of enclosure block 28 and extends between the enclosure block and the conductively bounded rectangular enclosure Hi8 connected to the output wave guide l'. The length, diameter and position of element lll are correlated so as to form with a portion of the interior of shield a quarter-wave transformer which serves to match the, impedance of the output cavity to the surge impedance of the coaxial coupling circuit.

The size of the output cavity 5tlg of Fig. 4 is varied. by moving the quarter-.wave transformer t@ along tliecoaxial line iii by means of an actuating tool passing through the holes 8l provided in dielectric washer S2 and extending down into the interior of block 28 to engage transformer lil. This movement is equivalent to moving both the slug le and member l of Fig. 24 together, It is readily apparent that the tuning arrangement employed in the Figs. 2 and 3 structures aords a much nner and smoother control of the out.` put cavity.

n radio-frequency trap circuit of the type emrioyed in the Fie. i modulator t0 prevent the es.- cape of the modulated microwave energy through the B+ plate supply line 38 is illustrated at Figs. 12 and 16, pages 1189 and 119i of the aforementioned article` entitled Microwave Converters by C. F. Edwards. The trap circuit is connected to the coaxial line near the anode of the tube in order to keep the plate load impedance lowl at the Sfr-megacycle signal frequency. Structurally, the trap circuit is characterized by a quarter-wave stub i Iii connected across the outer con.- ductor 'l1 of the coaxial coupling circuit and a quarter-wave coaxial line designated by Ill and H2. followed by a cylindrical polystyrene-filled resonant cavity H3 in the form of a disk transmission line shorted at its outer edge H4, The short circuit at the outer edge of the cavity is transferred to the gap H5 at the open end. of the quarter-wave stub thus effectively isolating the microwave output energy from the B+ plate supply line.

Placing the radio-frequency output trap in the coaxial line between the quarterwwave transformer te and the coaxial-to-wave-guide transducer 33 as is done in Fig. e tends to extend the lens-th of the coaxial line. '18. The output trap shown in Fig. 3 in the form of a quarter-wave wire 85 placed in the output wave guide is not only a sim.- pier structure than the trap cf Fig. 4, but it also affords a. saving of space and permits a compact unit since the length or" the coaxial line T8 can be made shorter.

Although the present invention has been described with reference to microwave modulating apparatus, it is apparent that various of the structural features are applicable to other forms of microwave apparatus such, for example, as microwave ampliers. Numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

l. In combination, a space discharge device having a grid septum and cathode and anode electrodes on opposite sides of the grid septum, a pair of conductively bounded space resonators having the grid septum in a common boundary portion thereof and each enclosing one of said electrodes, a coaxial coupling circuit comprising a tubular Outer conductor one end of which forms a part of the boundary of one of said space resonators and an inner conductor coupled to said one resonator, the separation between said coaxial conductors being constricted adjacent said one resonator over a length of substantially a quarter wavelength or odd multiple thereof at the resonance frequency ci' said one space resonator.

2. In combination, a space discharge device having a grid septum and cathode and anode electrodes on opposite sides of the grid septum, a pair of conductively bounded space resonators having the said grid septum in a common boundary portion thereof and each enclosing one of said electrodes, a coaxial coupling circuit comprising a tubular outer conductor one end of which forms a part of the boundary of one of said space resonators and an inner conductor coupled to said one resonator, a quarter-wave impedance transformer in said coupling circuit adjacent said one resonator and comprising an axially-movable section of one of said coaxial conductors whereby axial movement of said section adjusts th'e resonanc frequency of said one space resonator.

3. In combination, a space discharge device having a grid septum and cathode and anode electrodes on opposite sides of the grid septum, a pair of conductively bounded space resonators having the said grid septum in a common boundary portion thereof and each enclosing one of said electrodes, a coaxial coupling circuit comprising a tubular outer conductor one end of which forms a part of the boundary of one of said space resonators and an inner conductor coupled to said one resonator, an impedance transformer in said coupling circuit adjacent said one resonator, said impedance transformer comprising a section of one of said coaxial conductors that is axiallymovable and has a length of an odd number of quarter wavelengths at the resonance frequency of said one space resonator and a juxtaposed section of the other of said coaxial conductors that is axially-movable and has a length substantially greater than that of said rst-mentioned section whereby .axial movement of either of said sections affects the tuning of said one space resonator.

4. In combination, a conductively bounded enclosure adapted to support microwaves, a hollow stationary coupling probe entering through said enclosure, a tubular conductive shield around the portion of said probe external to said enclosure, a member within said shield adapted for movement axially of said probe, and an actuating tool passing through said enclosure and centrally through said hollow probe to engage said movable member and vary its position axially of said probe.

5. In combination, a conductively bounded enclosure adapted to support microwaves, a hollow stationary coupling probe one end of which enters through said enclosure, a tubular conductive shield around the portion of said probe external to said enclosure, an internally threaded tubular sleeve axially aligned with the other end of said hollow probe and constrained to be movable axially thereon and an actuating tool comprising a screw-driver or the like passing through said enclosure and centrally through said hollow probe to engage said threaded tubular sleeve and vary its position axially of said probe.

6. In combination, a conductively bounded enclosure adapted to support microwaves, a hollow stationary coupling probe one end of which enters through said enclosure, a cylindrical conductive shield around the portion of said probe external to said enclosure, a tubular sleeve axially aligned with the other end of said hollow probe and adapted for movement axially thereon, a member fixed within said tubular sleeve, said member having a threaded central section, a circularly-movable rod retained within said hollow probe, said rod having a threaded end to engage said threaded section of said member within said sleeve, a guide pin attached to the said other end of said probe and passing freely through a section of said member within said sleeve to prevent circular motion thereof, and an actuating tool comprising a screw-driver or the like passing through said enclosure and centrally through said hollow probe to engage said threaded rod and vary the position of said tubular sleeve axially of said probe.

7. In combination, an ultra-high frequency space discharge tube having a grid septum and a cathode and anode electrodes on opposite sides of the grid septum, a pair of conductively bounded resonant chambers having the said grid septum in a common boundary portion thereof and each enclosing one of said electrodes, a coaxial coupling circuit comprising a tubular outer conductor one end of which forms a part of the boundary of the one of said chambers that encloses said anode electrode and a hollow inner conductor, said coaxial coupling circuit including a quarter-wave impedance transformer adjacent said one chamber and comprising an axially-movable tubular conductor slidable on said hollow inner conductor and electrically connected to said anode electrode whereby an actuating tool may be inserted centrally through said hollow inner conductor to vary the position of said slidable tubular conductor axially to adjust the resonance frequency of said one chamber.

8. In combination, an ultra-high frequency space discharge tube having a grounded grid septum and a cathode and anode electrodes on opposite sides of the grid sep-tum, a pair of conductively bounded resonant chambers having the said grid septum in a common boundary portion thereof, a first one of said resonant chambers enclosing the said cathode electrode of said space discharge tube and the second enclosing the said anode electrode thereof, a Source of microwaves coupled to said rst resonant chamber for impressing waves of ultra-high frequency thereon, a coaxial coupling circuit comprising a tubular outer conductor one end of which forms a part of the boundary of said second resonant chamber and an inner conductor one end of which is connected to said anode electrode, and an axiallymovable quarter-wave coaxial impedance transformer in said coupling circuit adjacent said sec- 13 ond resonant chamber for adjusting the resonance frequency of said chamber.

9. A grounded grid microwave modulator comprising in combination, an ultra-high frequency space discharge tube having a grounded grid septum and a cathode and anode electrodes on opposite sides of the grid septum, a pair of conductively bounded resonant chambers having the said grid septum in a common boundary portion thereof, a first one of said resonant chambers enclosing the said cathode electrode of said space discharge tube and the second enclosing said anode electrode thereof, an input wave guide connected to said first resonant chamber, means for impressing carrier waves of microwave frequency on said first resonant chamber through said input wave guide, means for impressing signal waves on said cathode electrode for intermodulation with said carrier waves, filter means in said input wave guide to reflect the modulated side- 20 band energy appearing in said rst resonant 14 chamber into said second resonant chamber in phase with the modulated sideband energy appearing in said second chamber, a coaxial coupling circuit comprising a tubular outer conductor one end of which forms a part of the boundary of said second resonant chamber and an inner conductor one end of which is connected to said anode electrode, and an axially-movable quarterwave coaxial impedance transformer in said coupling circuit adjacent said second resonant chamber for adjusting the resonance frequency of said The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date Gethmann July 15, 1947 Number 

